In recent years, a GaN based high electron mobility transistor (HEMT), in which an AlGaN/GaN hetero junction is utilized and GaN is used as a carrier transit layer, has been actively developed. Gallium nitride is a material having a wide band gap, high breakdown field strength, and a large saturation electron velocity and, therefore, is a very promising material capable of realizing a large current, a high voltage, and a low on-resistance operation. Development to apply GaN to a next-generation high efficiency amplifier used in a base station and the like and a high efficiency switching element to control an electric power has been actively performed.
A dielectric breakdown voltage is an important parameter of a semiconductor device used as the high efficiency amplifier or the high efficiency switching element. The dielectric breakdown voltage is a maximum voltage, which can be applied between a source electrode and a drain electrode included in a semiconductor device. If a voltage exceeding the dielectric breakdown voltage is applied, the semiconductor device is broken. In particular, a semiconductor device serving as the high efficiency switching element to control an electric power is required to have a high dielectric breakdown voltage because several hundred volts of voltage is applied.
However, regarding a semiconductor device having the HEMT structure illustrated in FIG. 16, it is difficult to obtain a high dielectric breakdown voltage. In the semiconductor device having the HEMT structure illustrated in FIG. 16, an i-GaN layer 101, an AlGaN layer 102, and an n-GaN layer 103 are disposed sequentially on a substrate 100. Furthermore, in the semiconductor device having the HEMT structure illustrated in FIG. 16, a source electrode 104 and a drain electrode 105 are disposed on the AlGaN layer 102, and a gate electrode 106 is disposed on the n-GaN layer 103.
Regarding the semiconductor device having the HEMT structure illustrated in FIG. 16, several volts of voltage is applied to the gate electrode 106, and several hundred volts of voltage is applied to the drain electrode 105. Therefore, a potential difference between the drain electrode 105 and the gate electrode 106 is large, so that a large electric field is applied on a protective layer 107 disposed on the n-GaN layer 103. As for the protective layer 107, a SiN film is used in general. The dielectric breakdown voltage of the SiN film is low and; therefore, in the case where a large electric field is applied to the SiN film, the SiN film is broken. As a result, reduction in dielectric breakdown voltage of the whole semiconductor device occurs. Regarding the SiN film, film formation through thermal nitridation is difficult, and film formation is performed by a CVD method. The SiN film formed by the CVD method has poor film quality, so that the dielectric breakdown voltage of the SiN film is reduced. Like the SiN film, the dielectric breakdown voltage of a SiO2 film serving as an interlayer insulating film is low, so that in the case where a large electric field is applied to the SiO2 film, the SiO2 film is broken.
The potential of a wiring connected to the drain electrode 105 becomes very high. Consequently, potential differences between the wiring connected to the drain electrode 105 and a wiring connected to the source electrode 104 or the gate electrode 106 becomes large. As a result, it is necessary that the distances between the individual wirings are increased in order to prevent breakdown of the interlayer insulating films due to application of very high voltages to the interlayer insulating films between the individual wirings. Regarding the semiconductor device having the HEMT structure illustrated in FIG. 16, it is necessary to increase the distances between the individual wirings and; therefore, the flexibility in wiring is reduced, and an increase in chip area is provided.
For example, a method in which an improvement of dielectric breakdown voltage is attempted by disposing the gate electrode and the source electrode on the back of a substrate so as to increase the distance between the drain electrode and the source electrode has been known.
Related documents include Japanese Patent Laid-Open No. 2006-269939 and Japanese Patent Laid-Open No. 2007-128994.